Sudden strike 4 uses a game engine called Unity which is a cross-platform game engine developed by Unity Technologies. It is used to develop video games and simulations for computers, consoles and mobile devices. Currently, Sudden strike 4 (Beta) uses version 5.5.3p2 (You can check it by opening a file in "AssetBundles" folder with Wordpad).

In order to modify the game, it is necessary to first retrieve a tool which can extract different files such as textures, sounds, videos, models. This tool is called "Unity Assets Bundle Extractor". This tool (let's call it UABE) can extract and import files from unity archives. He can be found here.

V1.7 has been released with a TextAsset plugin fix and a new AudioClip plugin.
The AudioClip plugin can export Unity 5 AudioClips to uncompressed .wav files using FMOD. Basically it plays the sound to the .wav file in "non-realtime" mode.
See this post if you want to import sounds.

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If you want to add new sounds, you can create FMOD sound banks with the fsbank tool in the bin folder of the FMOD API
( http://www.fmod.org/download/ ). Only use one sound file at once. Press the build button and copy the .fsb file to the 7DaysToDie_Data folder.

You can export a dump of the AudioClip asset and change m_Source to the new file's name, set m_Offset to 0 and m_Size to the size of the new file in bytes you can lookup using the explorer. Then you can import the modified dump and save the .assets file.

I'm working on an AudioClip extractor but it probably won't work for vorbis files.